Shares in Toshiba fell more than 19 percent on Thursday, clocking a third day of heavy losses after the Japanese tech-to-nuclear conglomerate said earlier this week it faced a potential multi-billion dollar writedown. Earlier this week, Toshiba said cost overruns at a U.S. nuclear business it bought from Chicago Bridge & Iron last year, CB&I Stone & Webster, meant it could face "several billion dollars" in charges, acknowledging a bruising overpayment. Late on Wednesday, Moody's became the second agency to downgrade Toshiba's rating, pushing it deeper into "junk", or non-investment grade territory, with a Caa1 rating, from B3.